Compounds which decompose upon exposure to light to generate free radicals (free radical generating agents) are well known in the field of graphic arts. They have been widely used as photopolymerization initiators in photopolymerizable compositions, as photoactivators in free radical photographic compositions and as photo initiators for reactions catalyzed by acids formed by light. Using such free radical generating agents, various photosensitive materials useful for printing, duplicating, copying or other image formation are produced.
Organic halide compounds decompose upon exposure to light to generate halogen free radicals such as a chlorine free radical or a bromine free radical. These halogen free radicals are good hydrogen abstracting agents and form acids when a hydrogen donator is present. Application of such halide compounds to photopolymerization processes and free radical photographic processes have been described in Light-Sensitive Systems written by J. Kosar, pages 180 to 181 and 361 to 370, J. Wiley & Sons (New York, 1965).
As such compounds which generate halogen free radicals as a function of light, carbon tetrabromide, iodoform and tribromoacetophenone are typical examples which have been widely used heretofore. However, these free radical generating agents suffer from the drawback that they decompose only upon exposure to light in a limited wavelength range. Namely, they are sensitive to light in an ultraviolet range which is shorter than the main wavelength of the light sources generally used. These compounds have a poor free radical generating capability, because they do not have the ability of effectively utilize the light of near ultraviolet range to visible range.
For the purpose of overcoming this drawback, it has been proposed to expand the photosensitive wavelength range by adding certain kinds of sensitizers. There are, for example, sensitizers such as merocyanine dyes, styryl bases and cyanine bases, as disclosed in U.S. Pat. Nos. 3,106,466 and 3,121,633. Although the addition of these sensitizers expands the sensitive wavelength range of carbon tetrabromide or iodoform to the visible range, the result is still not satisfactory since it is difficult to select a sensitizer which has good compatibility with the free radical generating agents or other components in the photosensitive composition and has high sensitivity, although selection of the sensitizer having such compatibility is necessary.
In order to overcome this drawback, halogen free radical generating agents which have the photosensitive wavelength range in the near ultraviolet range to the visible range have been proposed. For example, halomethyl-s-triazine compounds are described in U.S. Pat. Nos. 3,954,475, 3,987,037 and 4,189,323. Although these compounds have a photosensitive wavelength range in the near ultraviolet range to visible range, their photo-decomposition sensitivities are relatively low since light exposed is not utilized effectively.